Many electronic devices include sub-circuits, and one sub-circuit may have a voltage requirement that is different from another sub-circuit. For example, laptop computers and cell phones, the power to which is primarily supplied by batteries, may have sub-circuits that require a voltage that is lower or higher than the battery supply voltage. Instead of providing a different power source for each sub-circuit to accommodate the sub-circuits' differing voltage requirements, DC-DC converters are often employed. DC-DC converters convert an input DC voltage to a different output DC voltage.
A DC-DC converter may provide an output voltage that is lower than its input voltage, in which case, the converter is generally referred to as a step-down converter. Buck converters are commonly used as step-down converters. FIG. 1 illustrates a single phase buck converter 100, which includes a control switching device 104 (often referred to as an upper switching device in a buck converter), a lower switching device 108, and an inductor 106. During a first interval, upper switching device 104 is turned on, coupling a source voltage 102 to inductor 106, while lower switching device 108 remains turned off. A voltage 113 (VL) is created across inductor 106, and current 111 passing through inductor 106 feeds a load 110, which is connected in parallel with a filtering capacitor 112.
Inductor 106 stores energy in the magnetic field created by current flowing through the inductor during this first interval. Then, during a second interval, upper switching device 104 is turned off, severing the connection of inductor 106 with source voltage 102; but, because of the energy stored in inductor 106 during the first interval, current 111 continues to flow through inductor 106 and lower switching device 108, feeding load 110.
Current 111 through inductor 106 decreases during the second interval because the inductor is disconnected from source voltage 102. If upper switching device 104 is turned back on during a third interval before the current through inductor 106 reaches zero, converter 100 is said to be operating in continuous conduction mode. If, on the other hand, the current through inductor 106 falls to zero for part of the second interval, converter 100 is considered to be operating in discontinuous conduction mode. It is known that when driving a light load, a buck type DC-DC converter operating in discontinuous conduction mode may provide higher efficiency than a buck type DC-DC converter operating in continuous conduction mode.
Attention is now directed to FIG. 2, which illustrates the current and voltage waveforms relating to buck converter 100 operating in continuous conduction mode. For simplicity, the timing diagram in FIG. 2 (and the remaining figures) depicts ideal components. When inductor 106 is coupled to input voltage 102 by turning switching device 104 on during the first interval, current 111 through inductor 106 increases along with the energy stored in inductor 106. Subsequently, when upper switch 104 is turned off during the second interval, the energy stored in inductor 106 begins to decrease, along with current 111 passing through inductor 106. During this second interval, as shown, voltage 113 (VL) across inductor 106 is equal to the negative of output voltage 115 (Vo). As the buck converter 100 is operating in continuous conduction mode, before the current 111 reaches zero, switch 104 is turned on again during a third interval. Thus, as in the first interval, current 111 begins to rise, and inductor 106 starts to store energy; the process is continually repeated to provide a voltage 115 (Vo) that is stepped down from the source voltage 102. Filtering capacitor 112 acts as a filter to level the output, and may also provide power to load 110 during such time that inductor 106 is not conducting current.
A duty cycle D of switch 104 is the ratio of the first interval time, and the sum of the times of the first and second intervals. For example, where switch 104 is turned on for 3 microseconds, and then turned off for 9 microseconds, the duty cycle of switch 104 is 0.25. For a given input voltage 102, output voltage 115 (Vo) of the buck converter 100 increases linearly with increasing duty cycle D of upper switch 104.
It is known to couple two or more DC-DC buck subconverters in parallel to output larger currents, as shown in FIG. 3, where two DC-DC buck subconverters 202a, 202b are connected in parallel to drive a common load in a two phase DC-DC converter 200. Each buck subconverter 202a, 202b is sometimes referred to as a “phase” of DC-DC converter 200 when each subconverter operates at a particular offset timing relationship with respect to the other subconverters of the converter. In such situations, buck converter 200 is typically referred to as a “multiphase” buck converter. Offset timing between phases of a multiphase converter is sometimes referred to as “phasing.”
Phase 202a has a control or upper switch 206a, a diode 208a, and an inductor 210a; phase 202b has a control or upper switch 206b, a diode 208b, and an inductor 210b. As can be seen from FIG. 3, the lower switch 108 (FIG. 1) in each phase 202a, 204a is replaced by diode 208a, 208b respectively, which decreases the cost and complexity of phases 202a, 202b, but may also lower their efficiency. Both phases 202a, 202b are tied at their respective inputs 212a, 212b to a voltage source 214. Outputs 216a, 216b of phases 202a, 202b respectively are both tied to a load 220, to which a filtering capacitor 222 is connected in parallel.
During a first interval, upper switch 206a of phases 202a is turned on, while upper switch 206b of phase 202b remains turned off. Current 224a passes through inductor 210a, thereby storing energy in a magnetic field of inductor 210a, and via output 216a, provides power to load 220. Then, during a second interval, upper switch 206a of phase 202a is turned off, severing the connection of inductor 210a with voltage source 214, while upper switch 206b of phase 202b is turned on, coupling inductor 210b to voltage source 214. Now, current 224b passes through inductor 210b, stores energy in its magnetic field, and provides power to load 220. Additionally, inductor 210a, by virtue of the energy stored in its magnetic field during the first interval, continues to provide power to load 220. However, the magnetic field of inductor 210a decays during this second interval, and consequently, current 224a passing through inductor 210a decreases. Next, during a third interval, either before or after current 224a through inductor 210a becomes zero, upper switch 206a is turned on again, while upper switch 206b is turned off, as in the first interval. Here, inductor 210a provides power to load 220 and stores energy in its magnetic field, while inductor 210b, which is now disconnected from source 214, continues to provide power to load 220 because of the energy that inductor 210b stored in its magnetic field during the second interval. In this way, all other things being equal, parallel buck phases 202a, 202b of FIG. 3 can provide a higher output current 221 to the load 220. Much like capacitor 112 of FIG. 1, filtering capacitor 222 of FIG. 3 is used to filter the ripple component at the output. FIG. 4 illustrates the current waveforms of currents 224a and 224b through inductors 210a, 210b respectively in relation to the state of upper switches 206a, 206b. 
In addition to providing a higher output current 221, this 2 phase converter 200 has several other advantages over the single phase buck converter 100 of FIG. 1. Notably, because of the parallel configuration and the off-set timing of upper switches 206a, 206b, frequency of the ripple component generated thereby is effectively doubled, while the total ripple current amplitude at output 215 is decreased due to ripple current cancellation. The capacitance 222 required to filter out this ripple component is decreased, which, in turn, significantly decreases the overall system cost. This likewise stands true for a buck converter having more than two phases, wherein frequency of the ripple component is further increased with correct phasing, and ripple cancellation is also more effective.
It is also known that when inductors of a two (or more) phase buck converter are magnetically coupled to some degree, efficiency of the buck converter is improved, and the ripple is reduced. This is explained in U.S. Pat. No. 6,362,986 to Schultz et al., which is incorporated herein by reference. FIG. 5 shows a two phase buck converter 300, which, except for a magnetic core 301 manifesting the coupling of inductors 310a, 310b, is generally similar to two phase buck converter 200 of FIG. 3. Subconverter or phase 302a has a control or upper switch 306a, a diode 308a, and an inductor 310a; subconverter or phase 302b has a control or upper switch 306b, a diode 308b, and an inductor 310b. Both phases 302a, 302b are tied at their respective inputs 312a, 312b to a voltage source 314. Outputs 316a, 316b of phases 302a, 302b respectively are both tied to a load 320, to which a filtering capacitor 322 is connected in parallel.
The ideal waveforms of currents 324a and 324b through inductors 310a, 310b respectively in relation to the state of upper switches 306a, 306b of the buck converter 300 operating in continuous conduction mode are illustrated in FIG. 6. As shown, magnetic coupling of inductors 310a, 310b causes the current flowing from a switching node 326a into inductor 310a to induce a current flowing from a switching node 326b into inductor 310b. Similarly, magnetic coupling of inductors 310, 310b causes current flowing from switching node 326b into inductor 310b to induce a current flowing from switching node 326a into inductor 310a. A person skilled in the art will appreciate that because of this doubling of the effective switching frequency in a two-phase buck converter having two coupled inductors as compared to a two-phase buck converter having two discrete inductors, the peak to peak inductor ripple current (and output voltage ripple) in the coupled inductor buck converter 300 will be half than that of a buck converter 200 using discrete inductors, thereby simplifying or reducing capacitor sizes in ripple filtering circuitry.